As well known to those skilled in the art, a remarkably wide range of fields, from an inspection of a semiconductor pattern of a few nm, to a human body of hundreds of mm requires a measurement of a three-dimensional shape, and various measurement methods are being developed.
Particularly, interest in three-dimensional shape measurement is mainly directed to measurements of objects in a size range of tens of μm to a few mm in manufacturing process management, wherein representative examples are a protrusion of a PDP in a flat display industry, the shape of a solder paste on a PCB substrate, the shape of a ball-shaped solder paste in a BGA (Ball Grid Array) in a surface mount technology, and so forth.
The manufacturing processes of the objects in the above size range are aimed to mass production. Thus, tests and measurements for the control of manufacturing processes have to be done on a real time basis. Now, almost every manufacturing process uses a measurement device or tester on a basis of a two-dimensional vision test.
As manufactured products become high in performance, a defect occurring in a manufacturing process, which a conventional two-dimensional tester cannot overcome, has abruptly increased. This enhances a need for a real time measuring device and tester on a basis of three-dimensions as a means to overcome the defect.
A most commonly used method for a real time three-dimensional measurement operates on the principle of optical triangulation. The optical triangulation calculates the level of a predetermined point on the object by analyzing varied positions of a beam of light projected onto the point. The calculation uses a geometric relationship of a triangular form consisting of a light source for irradiating a light beam onto the point, detection means for detecting the light beam and the point on the object.
In the three-dimensional shape measurement device using the optical triangulation, the light source and detection means must be set to predetermined angles with respect to an axis perpendicular to a plane on which the object is laid. The angles have to be included as factors in the calculation.
Various types of three-dimensional shape measurement systems have been developed using optical triangulation as a basic measuring principle. The three-dimensional shape measurement systems are classified into three application types as follows, according to a measurable quantity of data at one time.
The first application type is a system that measures the level of a point on the object at one time. In more detail, the first system projects a beam of point light such as a laser onto a point within a predetermined region on the object and then applies the above described optical triangulation to a detected position of a light spot varied with respect to the level of the point on the object, thereby achieving the level of the point on the object.
This system requires mechanical scanner means such as a galvanometer capable of scanning a light beam onto the region on the object, or optical means such as an acousto-optic modulator, in order to measure a level distribution in the region on the three-dimensional surface of the object. The system has a disadvantage in that a complex electronic device is required to synchronize the scanner means with measurement means so as to achieve a real time measurement by using scanner means. The system has another disadvantage in that the system requires a relatively long time to measure a region over tens of mm, as it can measure only one point at one time.
The second application type is a system that measures one line on the object at one time. The representative example of the second system may be a slit beam measurement system. The system transforms a point light beam such as a laser to a line-shaped light beam, thereby irradiating the line-shaped light beam onto the surface of the object. The line-shaped light beam projected onto the surface is deformed according to a level distribution along a line on the surface. The image of the deformed line-shaped light beam is obtained by detection means, and then optical triangulation is applied to each point on the line, whereby the level of each point on the line is calculated.
The third application type is a system that measures a predetermined two-dimensional region on the three-dimensional surface of the object at one time. The third system transforms a white light beam to a multi-colored light beam such as spectrum through a prism or filter, thereby irradiating the multi-colored light beam onto the region on the object. First, the multi-colored light beam is projected onto a reference flat surface, and then a color distribution or hue value distribution on the reference flat surface is stored, where the hue value is unique for each color.
Next, the multi-colored light beam is projected onto a region on the object having a certain level distribution. The hue values of points on the region of the object are distributed with respect to levels of the points on the object, whereby the hue value distribution of the region on the object is different from that of the reference flat surface. The above described optical triangulation is then applied to different positions of a point in the region on the object and of a point on the reference flat surface having the same hue value as that of the point in the region on the object, whereby the level of the point in the region on the object can be calculated.
Another example of the third application type is the system that projects a specific pattern made of various colored lights in the forms of bands, different from the multi-colored light beam such as spectrum, onto a predetermined region of the three-dimensional shape, thereby measuring the shape according to changes in the colored light bands. The system of this example uses the same principle as the slit beam measurement system. However, the system of the example projects a plurality of colored light bands, which are sorted by colors, onto the surface region of the object so as to measure the surface region, while the slit beam measurement system measures only one line on the object at one time.
The third application type has an advantage in that the system can measure a predetermined region on the three-dimensional surface of the object at a time, but this type has a disadvantage in that the system has to reduce the size of the region to implement repeated measurements on a micro meter scale. The measurements have to be repeated discontinuously, when the surface of the object is larger than the region on which a beam of light is projected. This problem makes the system not suitable for a real time tester in a practical manufacturing process of products being continuously produced.
In all of above described conventional systems for measuring a three-dimensional shape using optical triangulation, respective detection means must have a two-dimensional area on which a light beam is focused after being projected onto a point on the object. The coordinates of a position in the area, on which the light beam is focused, depend on the level of the point on the object, whether it is high or low. These coordinates are detected by the detection means, whereby a level information of the point on the object can be determined.
The detection means have disadvantages in that they must be set to have a predetermined angle with respect to an axis perpendicular to a plane on which the object is laid, and a shading effect occurs, thereby creating a region on the object wherein the detection means fails to focus. The detection means has further disadvantages in that an adjustment is necessary when the angle with which the detection means are set is changed, and so measurement factors are shifted. In short, the conventional system for measuring a three-dimensional surface of an object using optical triangulation has a disadvantage in that the adjustment is necessary whenever the angle with which the detection means are set is changed, thereby causing an inconvenience in usage and requiring a long time for measurement.